This invention relates to meander line loaded antennas in more particularly to a configuration of the meander line loaded antenna involving a cavity and embedding the antenna in the cavity, thereby permitting flush mount operation.
In the past, and as illustrated in U.S. Pat. No. 6,323,814 by John T. Apostolos, entitled Wideband Meander Line Loaded Antenna, assigned to the assignee hereof, and incorporated herein by reference, wide bandwidth miniaturized antennas can be provided through the utilization of planner conductors which are fed through a so-called meander line which involves impedance changes to reduce the physical size of the antenna while at the same time permitting wideband operation.
The plates of the meander line loaded antennas are configured to exist above a ground plane and are spaced therefrom, with a meander line connecting a top plate or element to the ground plane. For operation in the 225 MHz to 2 GHz range, the height of the plates which are spaced from the ground plane can exceed five inches. Were the meander line loaded antennas operate down to 100 MHz, then the height above the ground plane would be on the order of ten inches.
For vehicle top applications when using an above-the-ground plane meander line loaded antenna, a ten-inch or more dome would have to be employed on the car top which is both unsightly and which can increase turbulent flow behind the antenna at vehicle speeds.
When these antennas are utilized on supersonic aircraft, anything having hard edges and existing above the skin of the fuselage results in intolerable turbulence which cuts down the efficiency of the aircraft.
In the past, for aircraft operation, a flush-mounted crossed slot antenna has been utilized in which slots depend down into a cavity some five inches. However in the application the overall size of the antenna is 30xc3x9730 inches. As a result, these yard square antennas require a significant amount of real estate on the skin of the aircraft, which real estate is in short supply.
There is therefore need to provide a small wideband flush mount antenna which does not affect aircraft aerodynamics while at the same time providing the required wideband performance.
Whether for a cell phone, PCS, 802.11 and/or GPS application such as that which is required for either hand held wireless communication devices or for use in vehicle mounted apparatus, or for use in either satellite communications from an aircraft or for VHF communications from the aircraft to the ground, what is required is an exceedingly small flush mount antenna which has a wideband frequency response.
Such a wideband frequency response is possible with the apparatus described in U.S. Pat. No. 6,323,814 and more particularly in co-pending patent application Ser. No. 10/123,787, filed Apr. 16, 2002 assigned to the assignee hereof the incorporated herein by reference. In this patent application the low frequency cut off of the meander line loaded antenna is decreased due to a cancellation of the reactance of the antenna by the reactance of the meander line and parasitic capacitance.
It was not at all obvious that a meander line loaded antenna in which the plates of the antenna existed above a ground plane could be submerged in a conductive cavity. It was also not immediately obvious that one could obtain the reactance cancellation obtainable in an above-the-ground plane meander line loaded antenna when using any kind of cavity.
Note, when others have attempted to flush mount antennas, the size of the cavities involved were such to preclude their use due to the massive size of the cavity involved.
Also, it was not clear that the gain of the antenna at the zenith and horizon would match the same characteristics as those of an above-the-ground plane meander line loaded antenna, especially when in a loop mode. It will be appreciated that having a horizon gain that approximates that of the gain at the zenith is quite important for omnidirectional general coverage for the antenna. For instance, if one is in a vehicle and one wants coverage at the horizon where cell sites are located, then it is important that the gain in the horizontal direction be such as to robustly communicate with the cell sites.
Moreover, if the antenna is utilized in a GPS mode, it will be appreciated that the horizontal dilution of position is much smaller when signals comes from satellites at or near the horizon, as opposed to satellites which are directly overhead. Thus, the gain of the antenna towards the horizon is indeed a critical factor and one which could not be predicted from a meander line antenna with a plate above its ground plane.
Thus, it is important for flush mount applications to be able to replace the crossed-slot flush mount antenna which is a yard by a yard in area with one with considerably reduced dimensions. This type of real estate savings is indeed important not only in aircraft but also in terrestrial vehicles where appearance is important.
In the subject invention a flush-mounted meander line loaded antenna is identical in size and design to the meander line loaded antenna described above except for the location of the elements in a conductive cavity. As a result, the antenna is built at the top portion of the conductive cavity such that the top plates of the antenna are flush with a surrounding ground plane surface that meets the upper edge of the cavity. It is a feature of the subject invention that the meander line loaded antenna elements are at or below the plane of the conductive surface which carries the cavity. It is also important that the cavity volume be designed to be greater than 0.003 times the cube of the lowest frequency wavelength so as to guarantee maximum efficiency. It has been found that the subject cavity mounted antenna is governed by the Chu-Harrington relationship in which a form factor times Q, the quality factor, multiplied by the volume of the cavity divided by the cube of the wavelength in fact establishes maximum efficiency.
The way the cavity configuration is designed is to design the antenna conventionally and then having the dimensions of its top plates design a cavity whose volume is optimum as established by Chu-Harrington.
It will be appreciated that the Chu-Harrington relationship was developed for antennas which existed above a ground plane. It is the finding of the subject invention that a similar relationship holds for below ground plane antennas.
Moreover, it has been found that the gain at the zenith of the antenna and the gain at the horizon mimics exactly that of meander line loaded antennas in which the plates are above the ground plane.
What this means is that a flush mount antenna may be provided either for vehicles or aircraft, or indeed for handheld or portable devices such as laptop computers in which the antenna characteristics match those of prior meander line loaded antennas. These prior meander line loaded antennas are characterized by their small size and wideband characteristics. With the subject antenna, not only are these characteristics maintained, the flush mounting reduces the turbulent flow over the antennas, so they can be conveniently mounted on a vehicle or an aircraft.
Moreover, for wireless handsets and laptops these devices may be made thinner due to the fact that parts of the antenna may be submerged into the interior of the device. Additionally, antennas which are flush mounted in this manner are not easily broken off due to usage or mishandling.
It will be appreciated especially with regard to handheld wireless units that the whip antennas normally used are easily broken and is a cause major consternation for the user. Such breakage is avoided by the subject flush-mounted antennas. Likewise, for antennas which are mounted on relatively heavy devices such as laptops, placing them on a table or adjacent some other piece of equipment may result in damage to a surface-mounted antenna or the antenna may be broken off. Here again, the subject submerged antennas are not subject to this type of damage.
With the subject flush mount antenna, aside from the advantages of flush mounting, not only is the size minimized and the antenna characteristics maximized, the ruggedness of the unit is not compromised through the utilization of the antenna.
In summary, a wideband meander line loaded antenna is configured to be flush mounted to a conductive surface serving as a ground plane by embedding the meander line components within a conductive cavity surrounded at its top edge by the ground plane. The antenna thus looks out of a cavity recessed in the surface. By permitting flush mounting the meander line antenna, not only can the antenna dimensions be minimized due to the use of the meander line loaded antenna configuration, but in aircraft applications no part of the antenna exists above the skin of the aircraft, thereby to minimize turbulent flow. Moreover, when adapted to wireless handsets or laptop computers, the depth or thickness of the unit need not be increased when providing a wideband antenna, thus to minimize the overall dimensions of the device. Additionally, the flush mounted meander line antenna when utilized in the roof of a vehicle such as a car does not result in an unsightly protrusion from the top of the car, but rather is hidden in the recessed cavity, thereby permitting providing the vehicle with a wideband antenna which covers not only cellular frequencies but also the PCS band, the 802.11 band and GPS frequencies.